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A digital transmission service from a common carrier. Although developed in the 1960s and used internally, AT&T introduced it as a communications product to the public in 1983. Initially used for voice, its use for data grew steadily, and T1 and T3 lines were and still are widely used to create point-to-point private data networks. T-carrier lines use four wire cables. One pair is used to transmit; the other to receive.

The cost of the lines is generally based on the length of the circuit. Thus, it is the customer's responsibility to utilize the lines efficiently. Multiple lower-speed channels can be multiplexed onto a T-carrier line and demultiplexed (split back out) at the other end. Some multiplexors can analyze the traffic load and vary channel speeds for optimum transmission. See T1, T2, T3, DS, DSU/CSU and inverse multiplexor.